It is desirable to have an indication of the position of a rotary valve relative to its fully open and fully closed positions, both in order to know what function the valve is performing at any given time and to be able to control the valve. An electronic means for indicating the position is particularly desirable for valves that are controlled from a remote location or that are located in a place that is difficult to access. At the same time, it is desirable to have a mechanical indicator that can be read directly at the valve. Typically, a valve actuator is mounted on the same shaft as the valve and the valve position indicator (the shaft may be made in connected segments), and the actuator causes the shaft to rotate in order to open and close the valve. A controller at a remote location may control the actuator in order to open and close the valve while using the electronic position indicator to know the position of the valve as it is being rotated by the actuator.
Initial electronic valve position indicators relied on limit switches, mounted onto the actuator which operated the valve, to indicate the open and closed positions of the valve. Later valve position indicators included an indicator shaft operationally connected to the valve shaft or to the actuator shaft (which in turn is connected to the valve shaft), and mechanical devices rotating with the shaft to visually indicate the valve position. These later mechanical valve position indicators allow the indicator shaft to go straight through the valve position indicator, permitting the installation of a physical indicator on top of the valve position indicator (typically a window or a bubble on top of the indicator device secured to the top of the shaft, and a transparent cap or cover to protect this physical indicator and seal the housing) which rotates with the indicator shaft to give a clear visual indication of the valve position.
One prior art indicator, shown in U.S. patent application Ser. No. 11/202,227, “Kunz”, uses a magnetic flux sensor to detect a magnetic field induced by two opposed magnets mounted to rotate with the shaft of the valve position indicator. The polar axes of those magnets are perpendicular to and extend through the axis of rotation of the shaft. This sensor measures the magnetic field in a plane which is substantially coplanar with the magnets and on the axis of rotation of the shaft, so it locates the sensor directly above the shaft, which prevents the shaft from passing through to be able to use a mechanical valve-position indicator on top of the indicator device. This also prevents the device from using the enclosures which had been designed and certified for the previous generation of mechanical indicators.